Technical question about the two-byte address range

[F2Andy]

The documentation I have found says that with two-byte addresses, the range of addresses available is 1 to 10,239 (though some implementations might reduce that). The first two bits of the address byte have to be 1 to indicate this is a two-0byte address, which leaves 14 bits for the address itself, that gives you 16,383 addresses. However, The first eight bits cannot be all 1, as that is the null address, so that reduces the range by 256. That leaves 5,888 addresses unaccounted for.

 

Can anyone explain this? Have I misunderstood something?

 

I do appreciate this is just part of the magic that happens behind the scenes, and there is no reason I need to know this; I am just interested.

[AndrueC]

If the least signifcant byte cannot be 255 it can presumably still be 254 so that only 'costs' one address.  10,239 is an 11 bit address prefixed by 100.

 

10011111111

 

So :

1100000000001

to 

1110011111111

 

Still not using the full range though.

Edited May 15, 2022 by AndrueC

[Michael Hodgson]

Even if you were to win the euromillions jackpot and decide to build a layout with every loco in the BR fleet at any one time, I suspect you might not exceed the total.  How many locos are there in an Ian Allan combined ABC?

[Crosland]

The answer is in the Extended Packet Format Spec S-9.2.1 where it explains the addressing. Any address above 10000000 is followed by another byte to form a two-byte address, hence:

 

Addresses 11000000-11100111 (192-231)(inclusive): Multi Function Decoders with 14 bit addresses

 

So 14-bit (or extended or 4 byte or whatever you want to call them) addresses range from (stripping the top two bits, adding the second byte and switching to hex) 0x2000 - 0x27FF,  or 0 - 10,239. Some systems impose a limit of 0 - 9999 (hence refereed to as 4-digit).

 

[F2Andy]

Thanks for the responses. Building on comments here, I found this web page, which seems to be a more up-to-date version of S-9.2.1 than in on the NMRA site.

https://sites.google.com/site/markgurries/dcc-welcome-page/advanced-topics/decoder-addressing/decoding-dcc-addresses

 

The answer appears to be:

 

Quote

 

5) Addresses 11101000-11111110 (232-254)(inclusive):

Reserved for Future Use

 

 

[Nigelcliffe]

For DCC standards, its probably better now to be reading the "RailCommunity" documentation, rather than NMRA.  RailCommunity operates in German.   It seems that RailCommunity set things, then, after a while, NMRA adopts a translation.    The MERG site has translations of some of the RailCommunity documentation. 

 

- Nigel